The present invention relates to an image control device for an electrophotographic copier and, more particularly, to an image control device for reproducing a desirable image by compensating for a change in the surface temperature of a photoconductive element, which may result from a temperature characteristic of the photoconductive element.
As well known in the art, an electrophotographic copier uses a photoconductive element which is made up of an electroconductive base and a photoconductive insulating layer formed on the base. The photoconductor repeatedly undergoes sequential steps of charging, exposing, developing, transferring, clearing and charge dissipating, thereby producing a number of copies within a short period of time. Such photoconductors have a given temperature characteristic, more or less. Therefore, as the surface temperature of the photoconductor is elevated due to, for example, an elevation in ambient temperature in summer or frequent or continuous operation of the copier, the dark resistance is lowered to lower the surface potential and, thereby, the density of developed images, even if the amount of charge deposited on the photoconductor is constant. An implementation heretofore employed to eliminate the decrease in the photoconductor surface photential in such a situation is increasing a charging current to the photoconductor, that is, increasing a voltage applied to a charger to increase the amount of charge, in accordance with the elevation of the photoconductor surface temperature. However, exposure of the photoconductor to image light after such charging lowers the potential more in a dark area than in a light area due to the resulting elevation of the photoconductor temperature, so that lowering of the potential may be prevented in an image portion where the photoconductor surface potential is relatively high, but not in an image portion and background where it is relatively low. Therefore, while a desirable image may be reproduced when a document to be copied has a high image density, the image density and, therefore, the quality of the reproduced image is lowered when the document has a low image density or low contrast due to the decrease in the potential of low density image portions.